Determining the partial pressure of a gas, calibrating a pressure sensor

ABSTRACT

There is disclosed a method and system for determining the partial pressure of at least one gas in a mixture of gasses contained in a pressure vessel, the mixture being pressurised to a level which is above local atmospheric pressure. The method comprises the steps of positioning a gas analysis sensor ( 14 ) within a pressure vessel ( 10 ); exposing the sensor to the mixture of gasses at the pressure level found in the pressure vessel; operating the sensor to measure the actual partial pressure of the at least one gas in the mixture contained in the vessel; and periodically calibrating the sensor by directing a calibrating gas mixture ( 20, 22 ) to the sensor in the chamber, the calibrating gas mixture being breathable by a human being.

The present invention relates to a method and system for determining thepartial pressure of at least one gas in a mixture of gasses contained ina pressure vessel. The present invention also relates to a method ofcalibrating a gas analysis sensor used to measure the partial pressureof at least one gas in a mixture of gasses contained in a pressurevessel.

Divers operating at depth in a body of water experience elevatedpressures. On average, the pressure in a body of water increases by 1atmosphere (atm), or 1.01325 bar, for every 10 metres sea water (msw) indepth. Consequently, a diver operating at a depth of 100 msw willexperience a pressure of 20 atm (10.1325 bar), at 200 msw a pressure of20 atm (20.265 bar), and so on.

Under pressure at depth, increased volumes of gasses pass into solutionin a diver's body. This effect tends to occur at an exponential ratedepending upon the difference between the external gas partialpressures, the solubility of the breathed gasses and the partialpressures of the gases already dissolved in the body.

Divers have to ascend from depth slowly and, depending upon their depthand time of exposure to pressure, often have to stop at particulardepths during their ascent to the surface. These stops, coupled withslow ascent, enable the dissolved gasses to come out of solution throughthe lungs, and the differential pressures between the dissolved gases inthe diver's body and the external pressure is sufficiently low so thatdamaging bubbles do not form in the body.

When ‘saturation’ diving, divers have to remain at pressure for theentire duration of the dive, which may last for several weeks. Toaccomplish this, the divers live in pressurised chambers on the surface,and are transported to and from depth in a pressurised diving bell. Asingle process of stops and ascent occur at the end of the saturationdive. This is more efficient than doing a series of repeated shortduration dives, where the combined durations of decompression would bemuch greater.

Atmospheric air is a mixture of gasses, which typically containsapproximately 78% Nitrogen (N₂) and 21% Oxygen (O₂). The partialpressure of each gas in the mixture is the hypothetical pressure of thatgas if it alone occupied the volume of the mixture at the sametemperature. Accordingly, at a standard surface pressure of 1 atm, thepartial pressure of O₂ is typically 0.21 atm or ˜0.20265 bar. At a depthof 100 msw, the partial pressure of O₂ would therefore be 2.1 atm(˜2.0265 bar), and at 200 msw would be 4.2 atm (˜4.053 bar).

Divers operating at shallow depths for short periods employ a breathinggas which is often simply compressed air. However, divers descending togreater depths, and/or operating underwater for longer periods, have touse specialised breathing gas mixtures. This is because N₂ underpressure has a narcotic effect (commencing at a depth of around 40 m),and so Helium (He) is used. Also, O₂ is toxic at elevated partialpressures, depending upon the length of exposure and the partialpressure. For long term exposures, the partial pressure of O₂ must bekept to below 0.5 bar, and typically in the range of 0.44 to 0.48 barduring storage in a pressure chamber, and 0.40 to 0.60 bar for in-waterexcursions of a few hours. Carbon Dioxide (CO₂) must be kept to below0.005 bar, above which level the gas is toxic. Other gases may bepresent in trace amounts that should be measured, including CarbonMonoxide, residual Nitrogen and Argon.

Divers have to spend significant amounts of time under pressure in thewater and inside pressure chambers, during rest periods betweendeployment underwater, and also during subsequent decompression. Whenunder pressure, the constituents of divers' breathing gas have to bekept within accurate limits, in order to avoid the problems discussedabove. Since the partial pressure of the gasses is related to thepressure in the chamber and the proportion of the gas in the breathingmixture, at high pressures the actual proportion of gas (to be measured)in the mixture can be very small. For example, a diver operating at adepth of 100 msw (i.e. 10.1325 bar) for a sustained period will beprovided with breathing gas in which the partial pressure of O₂ at thatdepth will be kept to around 0.48 bar, as discussed above. As a result,at surface atmospheric pressure, the partial pressure of O₂ in themixture will be only 0.0474 bar by volume, which equates to just ˜4.74%of the total. For a depth of 200 msw (20.265 bar), the partial pressureof O₂ will be only 0.0237 bar, equating to just ˜2.37%, by volume, ofthe total. This presents a significant problem to the safe operation ofdiving systems, because the partial pressures of the gasses in thebreathing mixture must be accurately measured, so that the relativeproportions of the gasses can be accurately controlled.

In particular, the sensors have to be calibrated regularly, typicallytwice per day. Usually this is done with a two-point calibration, usingtwo special calibration gasses/gas mixtures that are fed to the sensor.“Zero gas” contains the inert gas that the divers are breathing, hasnone of the gas to be measured (in the mixture in the chamber), and isused to calibrate a zero point output of the analyser. “Span gas”contains the inert gas that the divers are breathing and has the desiredmaximum value of the gas to be measured in the mixture. This is used tocalibrate a desired maximum point output of the analyser. For example ata depth of 100 msw (10 bar) this might be 5% Oxygen with the balanceinert gas. This gives a partial pressure of 0.5 bar at that depth, whichis higher than the desired control limit but safe for the divers tobreathe.

The problem is that health & safety regulations dictate that onlybreathable gas mixtures are allowed inside the chamber. Consequently,“zero gas” cannot be supplied into the chamber to calibrate the sensor.This means that the sensor has to be located outside the chamber. Priorto passing through the sensor, the breathing gas mixture isde-pressurised to local atmospheric pressure. The difficulty here isthat the raw outputs from the sensors used to measure O₂ and CO₂ areproportional to the partial pressure of the gas in the mixture measuredat the sensor (at atmospheric pressure), and if proportional(percentage) readings are required corresponding to the partial pressureof that gas in the chamber, the partial pressure value has to factoredto eliminate the effects of pressure.

Using an extreme example of a working pressure equivalent to 300 msw (30atm or 30.3975 bar), and using the maximum allowable CO₂ partialpressure given above (0.005 bar), the measured partial pressure of CO₂in the breathing gas mixture at atmospheric pressure will be just0.00016 bar (or ˜0.016% by volume of the mixture total, at atmosphericpressure). The partial pressure band for oxygen would be measured to bejust 0.015 bar to 0.016 bar (or ˜1.48% to 1.58% O₂ by volume of themixture total, at atmospheric pressure). This is a significant problembecause, in order to accurately measure the proportions of CO₂ and O₂ inthe mixture, sensor resolution and accuracy needs to be superior to themagnitude of the quantity being measured. In addition, typical sensorsmay only have an accuracy of only around ±2%, with the result that theycannot accurately measure the proportions of CO₂ and O₂ in the mixture.

From the above, it can be seen that diver safety depends upon being ableto measure these partial pressures very accurately. It can also be seenthat the requirement for accuracy becomes more onerous as pressureincreases, and that for deep (high pressure) diving, very accurateinstruments are needed.

Whilst reference is made particularly to problems associated with diversoperating under pressure at depth, it will be understood that theproblems associated with working under pressure, and the safe operationof pressure chambers, is not restricted to divers. Many other workersoperate at pressure, including but not restricted to constructionworkers operating at elevated pressures in caissons and tunnels.Pressure chambers are also commonly used in the healthcare industry, fora variety of hyperbaric treatments.

It is amongst the objects of the present invention to obviate ormitigate at least one of the foregoing disadvantages.

According to a first aspect of the present invention, there is provideda method of determining the partial pressure of at least one gas in amixture of gasses contained in a pressure vessel, the mixture beingpressurised to a level which is above local atmospheric pressure, inwhich the method comprises the steps of:

-   -   positioning a gas analysis sensor within the pressure vessel;    -   exposing the sensor to the mixture of gasses at the pressure        level found in the pressure vessel;    -   operating the sensor to measure the actual partial pressure of        the at least one gas in the mixture contained in the vessel; and    -   periodically calibrating the sensor by directing a calibrating        gas mixture to the sensor in the chamber, the calibrating gas        mixture being breathable by a human being.

According to a second aspect of the present invention, there is provideda method of calibrating a gas analysis sensor used to measure thepartial pressure of at least one gas in a mixture of gasses contained ina pressure vessel, the mixture being pressurised to a level which isabove local atmospheric pressure, the method comprising the steps of:

-   -   positioning the sensor within the pressure vessel;    -   coupling the sensor to a calibrating assembly comprising a        source of a calibrating gas mixture which is breathable by a        human being, the source being located outside the vessel; and    -   selectively operating the calibrating assembly to supply the        calibrating gas mixture from the source to the sensor in the        vessel, to calibrate the sensor.

Advantageously, in the methods of the first and second aspect of thepresent invention, the use of a calibrating gas mixture which isbreathable by a human being enables the sensor to be positioned insidethe pressure vessel. In this way, the partial pressure measurement ofthe target gas (or gasses) can be carried out within the pressure vesselitself. Consequently, the sensor measures the actual partial pressure ofthe target gas (or gasses). This is in significant contrast to priormethods, in which the sensor is located outside of the pressure vessel,the pressure of the gas mixture to be analysed being reduced to localatmospheric level, and the partial pressure measurement then carried out(and subsequently factored to account for the measurement at thisreduced level, with consequent inaccuracies).

Reference is made herein to a calibrating gas mixture which is‘breathable’ by a human being. This should be taken to mean that thecalibrating gas comprises a mixture of gasses which, when breathed by ahuman being, is capable of supporting the life of that person.Accordingly, the gas mixture should be understood to comprise O₂; andnot to contain any gasses which would be toxic if breathed by a humanbeing and/or not to contain gasses in proportions which, at theprevailing pressure found in the pressure vessel, would be eithersubstantially immediately toxic to human beings or which would be toxicfollowing extended exposure (e.g. over a time period of the order ofseveral hours or days).

During calibration, the sensor may be operated to determine the partialpressure of at least one calibrating gas in the calibrating gas mixturedirected/supplied to the sensor.

The step of calibrating the sensor/calibration may comprise:

-   -   supplying a low point calibration gas mixture to the sensor        comprising a known first proportion of a calibration gas;    -   monitoring a partial pressure value of the calibration gas        outputted by the sensor;    -   comparing the partial pressure value of the calibration gas        outputted by the sensor to the actual partial pressure value        which the sensor should output for said proportion of        calibration gas at the pressure in the pressure vessel at which        the measurement is made;    -   determining any deviation between said values; and    -   calibrating the sensor to account for any such deviation.

The step of calibrating the sensor may comprise:

-   -   supplying a high point calibration gas mixture to the sensor        comprising a known second proportion of a calibration gas, the        second proportion being higher than the first proportion;    -   monitoring the partial pressure value of the calibration gas        outputted by the sensor;    -   comparing the partial pressure value of the calibration gas        outputted by the sensor to the actual partial pressure value        which the sensor should output for said proportion of        calibration gas at the pressure in the pressure vessel at which        the measurement is made;    -   determining any deviation between said values; and    -   calibrating the sensor to account for any such deviation.

It will be understood that the low point calibration gas mixture willtypically be supplied to the sensor first, followed by the high pointcalibration gas mixture. However, the high point calibration gas mixturemay be supplied first, followed by the low point calibration gasmixture. The references to first and second proportions of calibrationgas should be interpreted with this in mind. Specifically, in thisscenario, the high point calibration gas mixture will then contain aknown first proportion of calibrating gas, and the low point calibrationgas mixture will contain a known second proportion of calibrating gaswhich is lower than the first proportion.

The low point and high point calibration gas mixtures, being breathablegasses, will contain O₂ in known proportions. Suitably therefore, thecalibration gas is O₂. However, it will be understood that thecalibration gas may be any other constituent gasses of the mixtures. Thefirst and second proportions of the calibration gas may be selecteddepending upon the gas, and the pressure in the pressure vesselcontaining the sensor.

For example, where the calibration gas is O₂, the first proportion of O₂in the low point calibration gas mixture may be selected so that thepartial pressure of the gas in the calibration gas mixture, at thepressure in the vessel, is around 0.15 bar. This is equivalent tobreathing air at an altitude of around 5,500 m and is a safe low pointlevel for supporting human life. By way of example, at a pressure of 100msw in the vessel, an O₂ partial pressure of 0.15 bar would beequivalent to 0.015 bar at (typical) local atmospheric pressure, and soa proportion of 1.5% O₂ by volume in the mixture at atmosphericpressure. The second proportion of O₂ in the high point calibration gasmixture may be selected so that the partial pressure of the gas in thecalibration gas mixture, at the pressure in the vessel, is around 0.60bar. This is a safe high point level of O₂ that a human being can beexposed to for a period of several hours. By way of example, at apressure of 100 msw in the vessel, an O₂ partial pressure of 0.60 barwould be equivalent to 0.06 bar at (typical) local atmospheric pressure,and so a proportion of 6% O₂ by volume in the mixture at localatmospheric pressure.

The step of calibrating the sensor may comprise:

-   -   progressively varying the proportion of the calibrating gas in        the calibrating gas mixture directed to the sensor between the        first proportion and the second proportion;    -   monitoring the sensor output during the period that the        proportion of said calibrating gas is varied;    -   comparing the partial pressure values of the calibration gas        outputted by the sensor to the actual partial pressure values        which the sensor should output for said proportion of        calibration gas at the pressure in the pressure vessel at which        the measurement is made;    -   determining any deviation between said values; and    -   calibrating the sensor to account for any such deviation.

As is known in the industry, sensor deviation can occur both as a fixeddeviation (+/−) from the actual partial pressure, and as a ‘drift’ (+/−)which increases with pressure (and so resulting in a greater deviationat higher partial pressures than at lower partial pressures of thecalibration gas). Varying the partial pressure of the calibrating gas inthis way may therefore provide a dynamic indication of any sensor drift,so that an appropriate calibration can be made.

The step of calibrating the sensor may comprise directing a pre-preparedcalibration gas mixture to the sensor, said pre-prepared mixturecomprising a determined proportion of a calibrating gas. Where low pointand high point calibrating gas mixtures are directed to the sensor, twoseparate sources of pre-prepared mixtures may be directed to the sensor.The proportion of calibrating gas in the calibrating gas mixture may bevaried by controlling the flow of low and high point calibrating gasmixtures to the sensor.

The method may comprise preparing the calibrating gas mixture by mixingthe calibrating gas with at least one further gas, and progressivelyvarying the proportion of the calibrating gas in the mixture. This maybe achieved by providing a source of calibrating gas and a separatesource (or sources) of the at least one further gas, and mixing a flowof gas from the source of calibrating gas and a flow of gas from thesource of the at least one further gas, using flow control equipment.For example, the calibrating gas may be O₂ stored in a calibrating gascontainer and the further gas (providing the balance) may be He storedin a balance container.

A remainder (or balance) of the low and high point calibration gasmixtures may be an inert gas, in particular He, or a plurality of inertgasses of known proportions. The inert gas (or one of the inert gasses)may be employed as the calibration gas.

The step of positioning the sensor within the vessel may compriselocating the sensor within a container defining a sensor chamber, andarranging the sensor chamber so that it can communicate with the mixtureof gasses in the pressure vessel. The step of calibrating the sensor maycomprise flooding the sensor chamber with the calibration gas mixture toexpel all (or a majority) of the vessel mixture from the sensor chamber.In this way, the sensor is entirely (or substantially entirely) exposedto the calibrating gas mixture. As the calibrating gas mixture isbreathable, the calibrating gas mixture can be allowed to simply ventinto the pressure vessel.

The step of calibrating the sensor may comprise directing thecalibrating gas mixture into the vessel along a conduit having an outletwhich communicates with an interior of the vessel (optionally with thesensor chamber defined by the container), and arranging the outlet sothat it directs the mixture towards an inlet of the sensor. This mayserve to flood the sensor inlet with the calibrating gas mixture. Theoutlet may be arranged to direct a jet of calibrating gas mixturetowards the sensor inlet.

The method may comprise determining the partial pressures of a pluralityof gasses in the mixture of gasses in the vessel. The sensor may be usedto measure the partial pressures of a plurality of gasses, or aplurality of sensors may be employed, each sensor measuring the partialpressure of a single gas. Where there are a plurality of sensors, themethod may comprise periodically calibrating each of the sensors. Thesensors may be calibrated separately. Two or more sensors may becalibrated simultaneously. All of the sensors may be calibratedsimultaneously. Flow control equipment may control the flow ofcalibrating gas mixture to the desired sensor(s).

The mixture of gasses in the pressure vessel may be a breathable mixturecontaining CO₂, O₂ and which may contain a diluent gas such as He, andother trace gases such as Carbon Monoxide, Nitrogen and Argon at verysmall concentrations. The method may comprise determining the partialpressures of CO₂, O₂, trace gases and/or the diluent gas in the mixtureof gasses in the vessel.

The pressure vessel may be maintained at a substantially constantpressure over a determined period of time. Such may be the case wherethe pressure vessel is a pressure chamber providing life support fordivers for long periods of time (of the order of days), in betweenperiods where the divers operate underwater at depth. The pressurechamber may therefore be a life support pressure chamber, and may be adiving life support pressure chamber. The pressure chamber may be adiving bell.

The pressure chamber may additionally or alternatively function as adecompression chamber, in Which the pressure of the mixture of gasses inthe chamber is reduced over time in a controlled fashion, employing thedetermined partial pressure data. Thus the method may comprise reducingthe pressure of the mixture in the chamber over time, to decompress aperson or persons (e.g. divers) from the raised pressure in the chamber(which is set according to the pressure under which the person has beenoperating) to local atmospheric pressure.

The pressure vessel may be a gas storage tank or cylinder, in particulara diving gas storage cylinder. The method may comprise directing themixture of gasses in the cylinder to a diver, and may compriseperforming a reduction in the pressure of the mixture of gases directedfrom the cylinder to the diver to a level which is determined accordingto an operating depth of the diver.

Determination of the actual partial pressure of the at least one gas inthe mixture contained within the vessel (or other pressurised parts of apressurised system including the vessel) may enable determination of theproportion of said gas in the mixture. The method may comprisedetermining the partial pressure of a plurality of gasses in themixture, optionally of the gas or gasses making up a majority of themixture. This may facilitate determination of the composition of themixture of gasses contained in the vessel.

It will be understood that the calibrating step is typically carried outat determined time intervals, for example every 12 hours, especiallywhen the pressure vessel is providing a life support function.

According to a third aspect of the present invention, there is provideda method of controlling the partial pressure of at least one gas in amixture of gasses contained in a pressure vessel, the method comprising:

-   -   determining the partial pressure of the at least one gas in the        mixture following the method of the first aspect of the present        invention;    -   and then varying the proportion of the at least one gas in the        mixture in the vessel in order to maintain the partial pressure        of said gas within a target pressure range.

The method may be a method of providing life support for a personoperating under pressure (e.g. underwater), in which the mixture ofgasses in the chamber is a breathable mixture. The method may be amethod of decompressing a person from a pressure which is above localatmospheric pressure (at which the person has been operating e.g.underwater) down to local atmospheric pressure. The method may be amethod of providing life support for a person operating under pressure(e.g. underwater), in which the mixture of gasses is supplied, underpressure, to a person's breathing apparatus.

Further features of the step of determining the partial pressure of theat least one gas may be derived from the text set out above relating tothe first aspect of the invention.

According to a fourth aspect of the present invention, there is provideda pressure vessel comprising a gas analysis system for determining thepartial pressure of at least one gas in a mixture of gasses contained inthe pressure vessel, the mixture being pressurised to above localatmospheric pressure, in which the system comprises:

-   -   a gas analysis sensor positioned within the pressure vessel and        exposed to the mixture of gasses at the pressure level found in        the pressure vessel, the sensor being operable to measure the        actual partial pressure of the at least one gas in the mixture        contained in the vessel; and    -   a calibrating assembly for calibrating the sensor, the        calibrating assembly comprising a source of a calibrating gas        mixture which is breathable by a human being, the source being        located outside the vessel, in which the calibrating assembly is        operable to selectively supply the calibrating gas mixture to        the sensor in the vessel to calibrate the sensor.

The calibrating assembly may comprise a source of a low pointcalibration gas mixture, said mixture comprising a known firstproportion of a calibration gas. The calibrating assembly may comprise asource of a high point calibration gas mixture, said mixture comprisinga known second proportion of a calibration gas which is higher than thefirst proportion.

The calibrating assembly may be arranged

-   -   monitor a partial pressure value of the calibration gas        outputted by the sensor;    -   compare the partial pressure value of the calibration gas        outputted by the sensor to the actual partial pressure value        which the sensor should output for said proportion of        calibration gas at the pressure in the pressure vessel at which        the measurement is made;    -   determine any deviation between said values; and    -   calibrate the sensor to account for any such deviation.

The calibration assembly may comprise a device for comparing the partialpressure values, determining any deviation and calibrating the sensor.The device may comprise a processor.

The calibrating assembly may be operable to:

-   -   progressively vary the proportion of the calibrating gas in the        calibrating gas mixture directed to the sensor between the first        proportion and the second proportion;    -   monitor the sensor output during the period that the proportion        of said calibrating gas is varied;    -   compare the partial pressure values of the calibration gas        outputted by the sensor to the actual partial pressure values        which the sensor should output for said proportion of        calibration gas at the pressure in the pressure vessel at which        the measurement is made;    -   determine any deviation between said values; and    -   calibrate the sensor to account for any such deviation.

The calibration assembly may comprise a pre-prepared calibration gasmixture comprising a determined proportion of a calibrating gas. Wherelow point and high point calibrating gas mixtures are directed to thesensor, two separate sources of pre-prepared mixtures may be provided.

The calibration assembly method may be operable to prepare a calibratinggas mixture by mixing a calibrating gas with at least one further gas,and progressively varying the proportion of the calibrating gas in themixture. This may be achieved by providing a source of calibrating gasand a separate source (or sources) of the at least one further gas, andflow control equipment for mixing a flow of gas from the source ofcalibrating gas and a flow of gas from the source of the at least onefurther gas.

The system may comprise a container defining a sensor chamber in whichthe sensor is located, the sensor chamber arranged so that it is incommunication with the mixture of gasses in the pressure vessel. Thecalibration assembly may be arranged to flood the sensor chamber withthe calibration gas mixture to expel all (or a majority) of the vesselmixture from the sensor chamber.

The calibration assembly may comprise a conduit for directing thecalibrating gas mixture into the vessel, the conduit having an outletwhich communicates with an interior of the vessel (optionally with thesensor chamber defined by the container). The outlet may be arranged todirect the mixture towards an inlet of the sensor. The outlet may bearranged to direct a jet of calibrating gas mixture towards the sensorinlet.

The pressure vessel may be operable to maintain the pressure of mixtureat a substantially constant pressure over a determined period of time.The pressure vessel may be a pressure chamber, and may be a life supportchamber, e.g. for divers operating at depth for long periods of time (ofthe order of days). The pressure chamber may be a diving life supportpressure chamber. The pressure chamber may be a diving bell. Thepressure chamber may additionally or alternatively function as adecompression chamber, in which the pressure of the mixture of gasses inthe chamber can be reduced over time in a controlled fashion, employingthe determined partial pressure data. The pressure vessel may be a gasstorage cylinder or tank, in particular a diving gas storage cylinder.

According to a fifth aspect of the present invention, there is provideda gas analysis system for determining the partial pressure of at leastone gas in a mixture of gasses contained in a pressure vessel, themixture being pressurised to above local atmospheric pressure, thesystem having the features of the system which forms part of thepressure vessel of the fourth aspect of the invention.

Further features of the gas analysis system may be derived from aboverelating to the fourth aspect of the invention.

According to a sixth aspect of the of the present invention, there isprovided a system for calibrating a gas analysis sensor used to measurethe partial pressure of at least one gas in a mixture of gassescontained in a pressure vessel, the mixture being pressurised to a levelwhich is above local atmospheric pressure, the gas analysis sensor beingpositioned within the pressure vessel so that it is exposed to themixture of gasses at the pressure level found in the pressure vessel,the sensor being operable to measure the actual partial pressure of theat least one gas in the mixture contained in the vessel, in which thesystem comprises:

-   -   a calibrating assembly for calibrating the sensor, the        calibrating assembly comprising a source of a calibrating gas        mixture which is breathable by a human being, the source being        located outside the vessel, the calibrating assembly being        operable to selectively supply the calibrating gas mixture to        the sensor in the vessel to calibrate the sensor.

Further features of the system may be derived from or with respect toany of the aspects of the invention set out above.

Embodiments of the present invention will now be described, by way ofexample only, with reference to the accompanying drawings, in which:

FIG. 1 is a schematic illustration of a pressure chamber with a gasanalysis system of a type known in the art and which is employed todetermine the partial pressure of a gas in a mixture of gases containedin the pressure chamber;

FIG. 2 is a schematic illustration of a pressure vessel, in the form ofa pressure chamber, comprising a gas analysis system according to anembodiment of the present invention;

FIG. 3 is a graph illustrating output voltage readings (V_(o)) of asensor forming part, of the gas analysis system of FIG. 2, indicative ofa measured partial pressure of a target gas, against actual partialpressure of the gas, the graph showing possible deviations and driftfrom the actual partial pressure indicative of a sensor calibrationrequirement;

FIG. 4 is a graph similar to FIG. 3 illustrating voltage output readings(V_(o)) of the sensor against actual partial pressure of the gas, withsafe and comfortable partial pressure ranges (for users of the pressurechamber) superimposed;

FIG. 5 is a schematic illustration of a pressure vessel, in the form ofa gas storage cylinder coupled to a diver's breathing apparatus, andhaving a gas analysis system according to an embodiment of the presentinvention; and

FIG. 6 is a schematic illustration of a pressure vessel, in the form ofa gas storage cylinder coupled to a diving bell, and having the gasanalysis system of FIG. 5.

Turning firstly to FIG. 1, there is shown a schematic illustration of apressure chamber 1 incorporating a gas analysis system 2 of a type whichis known in the art. The pressure chamber 1 is a life support chamber,containing a breathable mixture of gases at a pressure which is abovelocal atmospheric pressure. The pressure chamber 1 provides life supportfor a person or persons who have been exposed to elevated pressures(above local atmospheric pressure) for relatively long periods of time,of the order of hours or days. Suitable examples include diversoperating underwater at depths of up to around 200 msw, or even 300 mswwith specialised diving equipment. The pressure chamber 1 can also beused as a decompression chamber, during subsequent decompression of aperson or persons exposed to such elevated pressures.

The prior gas analysis system 2 comprises a pressure reducing valve 3,throttle valve 4, flow meter 5, gas analysis sensor 6, flow line 7 andpressure measurement device 8. The gas analysis system 2 is coupled tothe pressure chamber 1 so that a portion of the mixture of gases can beexhausted from the chamber through the flow line 7, for direction to thesensor 6 for analysis. The pressure reducing valve 3 reduces thepressure of the mixture of gases directed to the sensor 6 to localatmospheric pressure level. The throttle valve 4 serves for throttlingthe flow of the mixtures of gases to a flow rate suitable for the gasanalysis sensor 6, the flow being metered using the flow meter 5, toverify the flow rate is within a suitable range. The sensor 6 isresponsive to partial pressure, and the output of the sensor isproportional to the percentage of the target gas present at atmosphericpressure. The pressure measurement device 8 is for determining thepressure inside the chamber 1, so that partial pressure inside thechamber can be calculated. The system 2 suffers from the significantdisadvantages discussed above, in terms of the accuracy of the partialpressure measurement which is taken. This has a consequent impact uponthe accuracy of the partial pressure of the gas in question at elevatedpressure in the chamber 1, which is determined employing the measuredpartial pressure, factoring in the chamber pressure, as described indetail above.

Features of methods, systems and pressure vessels according to aspectsof the present invention will now be described with reference to FIGS. 2to 6 of the accompanying drawings.

Thus turning now to FIG. 2, there is shown a schematic illustration of apressure vessel in the form of a pressure chamber 10, having a gasanalysis system indicated generally by reference numeral 12, accordingto an embodiment of the present invention. As with the prior chamber 1and gas analysis system 2 of FIG. 1, the pressure chamber 10 provides alife support/decompression function, particularly for a diver. The gasanalysis system 12 serves for determining the partial pressure of atleast one gas in a mixture of gases contained in the pressure chamber10, where the mixture is pressurised to above local atmosphericpressure.

The system 12 comprises a gas analysis sensor 14 which is responsive topartial pressure and which is located within the pressure chamber 10.Specifically, the sensor 14 is located within an internal void 16defined by a wall 18 of the chamber 10. The sensor 14 is exposed to thebreathable mixture of gases contained within the chamber 10, in the void16, which are at the elevated pressure level. Since the sensor 14 islocated within the chamber 10, it measures the actual partial pressureof the target gas, at the pressure of the mixture prevailing in thechamber. This overcomes difficulties associated with prior systems, suchas the system 2 of FIG. 1, described above. In particular, measuring thepartial pressure of the target gas at the elevated pressure level foundwithin the chamber 10 avoids the inaccuracies which occurred in theprior system 2, due to the requirement to position the sensor 6 outsidethe chamber 1, and to reduce the pressure of the mixture of gases beinganalysed to local atmospheric pressure prior to measurement at thesensor 6.

The ability to locate the sensor 14 within the pressure chamber 10, andso to measure the actual partial pressure of the target gas, isfacilitated by the gas analysis system 12. This is because the system 12is arranged to periodically calibrate the sensor 14 by directing acalibrating gas mixture 20, 22 to the sensor 14 within the chamber 10,the calibrating gas mixture being one which is breathable by a humanbeing. Previously and as described in relation to the prior gas analysissystem 2 shown in FIG. 1, it was necessary to locate the sensor 6outside the chamber 1 so that calibration could be effected. This wasdue to the use of a zero gas which was not breathable, in that itentirely comprised the inert gas that persons inside the chamber 1 werebreathing (typically He). Thus by providing a calibrating gas mixturewhich is breathable, this allows the sensor 14 to be located within thepressure chamber 10.

It will be understood that the calibrating gas mixture is breathable inthat it is capable of supporting the life of persons within the chamber10. The calibrating gas mixture comprises O₂ and is typically a mixtureof O₂ with a suitable inert balance gas such as helium, the mixture thusbeing “Heliox”, which is well known in the industry. As will bedescribed in more detail below, the proportion of O₂ in the calibratinggas mixture is carefully controlled.

Whilst reference is made herein to a method of determining the partialpressure of at least one gas in a mixture of gases contained in thepressure vessel 10, and to a corresponding system 12, the presentinvention also encompasses a method of calibrating a gas analysis sensorand a system for calibrating the sensor 14.

The systems and methods of the present invention will now be describedin more detail.

As will be understood by person skilled in the art, gas analysis sensorssuch as the sensor 14 shown in FIG. 2 can provide partial pressureoutputs suffering from one or both of fixed deviations (+/−) from theactual partial pressure of the target gas, and a phenomenon known as“drift” (+/−) in which a deviation from the actual partial pressureincreases with pressure (resulting in a greater deviation at higherpartial pressures than at lower partial pressures of the target gas).

In order to effectively ascertain both fixed deviations and drift,calibration of the sensor 14 is achieved by separately supplying a lowpoint calibration gas mixture 20 to the sensor 14, and a high pointcalibration gas mixture 22 to the sensor. The low point calibration gasmixture 20 comprises a known first proportion of a calibration gas,which may be any suitable constituent of the gas mixture, but which mayparticularly be O₂. The high point calibration gas mixture 22 comprisesa known second proportion of the calibration gas, the second proportionbeing higher than the first proportion.

The result of this is that the partial pressure of the calibration gasin the low point gas mixture 20, at the pressure prevailing in thechamber 10, is lower than the corresponding partial pressure of thatcalibration gas in the high point calibration gas mixture 22, Since thefirst and second proportions of the calibration gas in the low and highpoint mixtures 20 and 22 is known and indeed carefully controlled, theactual partial pressures of the calibration gas which should beoutputted by the sensor 14 are known. Any deviations or drift of thecalibration gas partial pressures outputted by the sensor 14 cantherefore be determined and the sensor calibrated appropriately.

For example and turning to FIG. 3, there is shown a graph which is aplot of voltage output of the sensor 14 (V_(o)) against partial pressure(P) of the calibrating gas, which is suitably O₂ as discussed above (fora static partial pressure of the mixture containing the target gas).

The voltage output V_(o) of the sensor 14 directly corresponds to thepartial pressure of the gas being measured by the sensor (which undernormal circumstances is a constituent of the mixture in the chamber 10,but during calibration is a constituent of the calibration gas mixture).The partial pressure P of the target gas is the actual partial pressurethat the sensor should output for a particular, static pressure of thegaseous mixture being analysed. During calibration, the proportion ofthe target gas in the calibration gas mixture, and so the partialpressure P of the target gas, is known. In the illustrated example, thestatic pressure is that which would be experienced at 100 msw and so is10 atm (10.325 bar).

The line 24 is a plot of the voltage V_(o) which a correctly functioningsensor 14 should output, as the partial pressure of the target gas (andthus the proportion of gas in the calibration gas mixture) increasesfrom 0 to an upper level P_(u). The line 26 is a plot of the voltageV_(o) outputted by the sensor 14 which is indicative of a deviation ‘d’from the actual partial pressure of the target gas (and thus from theline 24). The line 28 illustrates a situation in which the sensor 14 isexperiencing a drift which increases with partial pressure of the targetgas, the slope of the line 28 being different to that of the line 24. Ascan be seen, at the upper pressure P^(u), the drift has resulted in adeviation ‘D’ in the partial pressure outputted by the sensor 14,compared to the actual partial pressure of the target gas (as indicatedby the line 24).

This is best explained as follows, A correctly functioning sensor 14, asindicated by the line 24, outputs a voltage V₁ for a particular partialpressure P₁ of the target gas (say O₂). When the sensor 14 isexperiencing a fixed deviation d (line 26), a voltage V₂ is actuallyoutputted by the sensor, which corresponds to a partial pressure P₂which is higher than the actual partial pressure P₁. In other words, thesensor 14 is providing a false higher indication of the partial pressureof O₂ than is actually the case. When the sensor 14 is experiencing adrift resulting in a deviation 1) (line 28), a voltage V₃ is actuallyoutputted by the sensor, which corresponds to a partial pressure P₃which is lower than the actual partial pressure P₁. In other words, thesensor 14 is providing a false lower indication of the partial pressurethan is actually the case. It will be understood that this can becritical to the safety of persons in the chamber 10, when the sensor 14is functioning to provide partial pressure measurements of O₂ (or othergasses) in the mixture in the chamber 10.

Supply of the low and high point calibration gas mixtures 20 and 22 tothe sensor 14 is controlled by the gas analysis system 12. To this end,the system 12 comprises sources of low and high point calibrating gasmixtures in the form of high pressure storage cylinders 30 and 32.Valves 34 and 36 are associated with the respective cylinders 30 and 32,to select a gas output from the storage cylinders. Pressure regulators38 and 40 are coupled to the respective valves 34 and 36 and, inconjunction with a flowmeter 44, control the flow and pressure ofcalibrating gas mixtures 20, 22 from the cylinders 30, 32. Typically,the pressure of the mixtures 20 and 22 will be controlled to be slightlyabove the pressure of the gaseous mixture in the chamber 10 (to providea positive flow into the chamber). The calibrating gas mixture 20, 22flows to the sensor 14 along a conduit 42, checked by the flowmeter 44.

Typically, the low point calibration gas mixture 20 will be supplied tothe sensor 14 first, followed by the high point calibration gas mixture22. As described above, suitably the calibration gas forming part of thelow and high point calibration gas mixtures 20 and 22 will be O₂. Theproportion of O₂ in the low point calibration gas mixture 20 is selectedso as to provide a partial pressure of O₂, at the pressure of themixture in the chamber 10, of around 0.15 bar. This is equivalent tobreathing air at an altitude of around 5,500 meters and is a safe lowpoint for supporting human life. This is illustrated in FIG. 4, which isa graph similar to FIG. 3, where the low point level 46 is shown on theline 24. For measurements at 100 msw, the low point partial pressure O₂of 0.15 bar is equivalent to 0.015 bar at local atmospheric pressure,and so a proportion of 1.5% O₂ by volume in the calibration gas mixture20 at atmospheric pressure.

In the high point calibration gas mixture 22, a safe high point level ofO₂ is selected to provide a partial pressure O₂ of around 0.60 bar atthe pressure of the mixture in the chamber 10. A partial pressure O₂ of0.60 bar is above a target range of 0.44 to 0.48 bar, but as describedabove, is acceptable for in-water excursions of a few hours and sorepresents a safe high point level 48.

The impact that a deviation d (line 26) or a drift resulting in adeviation D (line 28) in the sensor 14 output would have is illustratedin FIG. 4. In particular, a safe partial pressure range R₁, between thelow and high point levels 46 and 48 is shown in the graph. This range R₁encompasses a narrower pressure range R₂, which represents a morecomfortable operating range of partial pressure O₂ for persons in thechamber 10.

A deviation d in the partial pressure O₂ outputted by the sensor 14,indicated by the line 26, will result in a partial pressure readingbeing outputted which is a significant magnitude higher than the actualpartial pressure of O₂ in the chamber 10. This is critical at the lowerend of the range R₁, because the sensor 14 could be indicating a partialpressure of O₂ which is at or above the low point level 46, but which isactually lower and insufficient to support persons in the chamber 10.

A drift resulting in a deviation D in the partial pressure O₂measurement outputted by the sensor 14, indicated by the line 28, wouldin contrast result in a partial pressure reading being outputted whichis a significant magnitude lower than the actual partial pressure of O₂in the chamber 10. This is critical at the upper end of the range R₁,because the sensor 14 could be indicating a partial pressure of O₂ whichis at or below the high point level 46, but which is actually higher andtoxic to persons in the chamber 10.

In the illustrated embodiment, the system 12 comprises a container 50which defines a sensor chamber 52 in which the sensor 14 is mounted. Thesensor chamber 52 can communicate with the mixture of gases in thepressure chamber 10, contained in the void 16, suitably by way ofapertures 54 in a wall 56 of the container 50. During calibration of thesensor 14, the sensor chamber 52 is flooded with the selectedcalibration gas mixture 20, 22, thereby expelling all (or a majority) ofthe gaseous mixture present in the chamber 10 from the sensor chamber52. In this way, the sensor 14 is entirely or substantially entirelyexposed to the selected calibrating gas mixture 20, 22. As the sensorchamber 52 is open to the chamber 10, the selected calibrating gasmixture 20, 22 (which is breathable) simply vents in to the chamber 10through the apertures 54.

The conduit 42 along which the calibration gas mixture 20, 22 flows hasan outlet 58 which communicates with the sensor chamber 52. The outlet58 is arranged relative to an inlet 60 of the sensor 14 so that itdirects the calibration gas mixture 20, 22 directly towards the sensorinlet 60. This may assist in flooding the sensor inlet 60 with thecalibrating gas mixture 20, 22 and may enable the container 50 to bedispensed with.

The gas analysis system 12 comprises a device for comparing the partialpressure values, determining any deviation and automatically calibratingthe sensor, the device indicated generally by reference numeral 62 andtaking the form of a suitable processor. The processor 62 includes adisplay 64 for displaying the partial pressure measurements of thetarget gas outputted by the sensor 14. Automatic calibration of thesensor 14 is achieved by suitable software carried on the processor 62,of a type which is readily available in the industry.

Turning now to FIG. 5, there is shown a schematic illustration of apressure vessel and gas analysis system in accordance with anotherembodiment of the invention. The pressure vessel is designated byreference numeral 100, and takes the form of a gas storage cylinder ortank, which stores breathing gas for a diver 66. In this embodiment, thegas analysis system is indicated generally by reference numeral 112.Like components of the system 112 with the system 12 of FIG. 2 share thesame reference numerals, incremented by 100.

The system 112 is essentially of like construction and operation to thesystem 12. The substantial difference between the embodiment of FIG. 5and that of FIG. 2 is that, instead of monitoring the partial pressureof a target gas or gases within a pressure chamber suitable forreceiving e.g. a diver 66, the system 112 monitors the partial pressureof a gas/gases in a mixture contained in the tank 100, which aresupplied to breathing apparatus 68 worn by the diver 66 via a hose 70.The system 112 is used to test the breathing gas before it goes to thediver 66 in the water.

The breathing gas is stored in the tank 100, which is a high pressurecylinder, and the pressure is reduced by a pressure control device 72,in the form of a pressure regulator. This provides overpressure withrespect to the hydrostatic pressure at the particular operating depth ofthe diver 66, matched to the requirement of the breathing apparatus 68that the diver is using. As can be seen from the drawing, in thisinstance, a sensor 114 is provide in the tank 100, and the low and highpoint calibration gas mixtures 120 and 122 stored in respectivecylinders 130 and 132. Valves 134 and 136 and regulators 138, 140,together with a flow meter 144, control the flow of the low and highpoint calibration gas mixtures 120, 122 to the sensor 114 to calibratethe sensor. Monitoring and calibration of the sensor 114 is performed bya processor 162, and partial pressures of target gas output on a display164.

FIG. 6 shows a variation of the system 112 shown in FIG. 5, in which themixture of gases in the tank 100 is supplied to a diving bell 74(through the hose 70), which provides life support for the diver 66during deployment underwater. For example, the bell 74 can be used fortransferring the diver 66 from a pressure chamber at surface (thepressure of the breathing mixture in the bell 74 being set at the samelevel as that in the chamber), as well as for providing underwater lifesupport for the diver during times between excursions out of the belland into the water.

Typically, the hose 70 is coupled to a gas panel 76 in the bell 74,which controls the supply of breathing gas both to an interior of thebell 74, and through a hose 78 to the breathing apparatus 68 worn by thediver 66. The gas panel 76 is also coupled to emergency breathing gascylinders (not shown) via hoses 80 and 82, for supplying breathing gasto the interior of the bell 74 and/or to the diver 66 (via the hose 78)in the case of an emergency loss of supply of gas from the tank 100 atsurface. From the above, it will be appreciated that the system 112 mayeffectively serve for monitoring the breathing gas supplied to both thebell 74 and to the diver's breathing apparatus 68.

Various modifications may be made to the foregoing without departingfrom the spirit or scope of the present invention.

The invention claimed is:
 1. A method of determining the partialpressure of at least one gas in a mixture of gasses contained in apressure vessel, the mixture being pressurised to a level which is abovelocal atmospheric pressure, in which the method comprises the steps of:positioning a gas analysis sensor within the pressure vessel; exposingthe sensor to the mixture of gasses at the pressure level found in thepressure vessel; operating the sensor to measure the actual partialpressure of the at least one gas in the mixture contained in the vessel;and periodically calibrating the sensor by directing a calibrating gasmixture to the sensor in the vessel, the calibrating gas mixture beingbreathable by a human being.
 2. A method as claimed in claim 1, in whichthe step of positioning the sensor within the vessel comprises locatingthe sensor within a container defining a sensor chamber, and arrangingthe sensor chamber so that it communicates with the mixture of gasses inthe pressure vessel.
 3. A method as claimed in claim 2, comprisingdirecting the calibrating gas mixture into the vessel along a conduithaving an outlet which communicates with the sensor chamber, andarranging the outlet so that it directs the mixture towards an inlet ofthe sensor.
 4. A method as claimed in claim 3, comprising arranging theoutlet so that it directs a jet of calibrating gas mixture towards thesensor inlet.
 5. A method of determining the partial pressure of atleast one gas in a mixture of gasses contained in a pressure vessel, themixture being pressurised to a level which is above local atmosphericpressure, in which the method comprises the steps of: positioning a gasanalysis sensor within the pressure vessel; exposing the sensor to themixture of gasses at the pressure level found in the pressure vessel;operating the sensor to measure the actual partial pressure of the atleast one gas in the mixture contained in the vessel; and periodicallycalibrating the sensor by directing a calibrating gas mixture to thesensor in the vessel, the calibrating gas mixture being breathable by ahuman being, wherein the calibration gas mixture comprises a knownproportion of a calibration gas.
 6. A method as claimed in claim 5, inwhich the step of calibrating the gas mixture comprises: monitoring apartial pressure value of the calibration gas outputted by the sensor;comparing the partial pressure value of the calibration gas outputted bythe sensor to the actual partial pressure value which the sensor shouldoutput for said proportion of calibration gas at the pressure in thepressure vessel at which the measurement is made; determining anydeviation between said values; and calibrating the sensor to account forany such deviation.
 7. A method as claimed in claim 5, in which thecalibration gas is O₂.
 8. A method of determining the partial pressureof at least one gas in a mixture of gasses contained in a pressurevessel, the mixture being pressurised to a level which is above localatmospheric pressure, in which the method comprises the steps of:positioning a gas analysis sensor within the pressure vessel; exposingthe sensor to the mixture of gasses at the pressure level found in thepressure vessel; operating the sensor to measure the actual partialpressure of the at least one gas in the mixture contained in the vessel;and periodically calibrating the sensor by directing a calibrating gasmixture to the sensor in the vessel, the calibrating gas mixture beingbreathable by a human being, in which the step of calibrating the sensorcomprises: supplying a low point calibration gas mixture to the sensorcomprising a known first proportion of a calibration gas; monitoring apartial pressure value of the calibration gas outputted by the sensor;comparing the partial pressure value of the calibration gas outputted bythe sensor to the actual partial pressure value which the sensor shouldoutput for said proportion of calibration gas at the pressure in thepressure vessel at which the measurement is made; determining anydeviation between said values; and calibrating the sensor to account forany such deviation.
 9. A method as claimed in claim 8, in which the stepof calibrating the sensor comprises: supplying a high point calibrationgas mixture to the sensor comprising a known second proportion of acalibration gas, the second proportion being higher than the firstproportion; monitoring the partial pressure value of the calibration gasoutputted by the sensor; comparing the partial pressure value of thecalibration gas outputted by the sensor to the actual partial pressurevalue which the sensor should output for said proportion of calibrationgas at the pressure in the pressure vessel at which the measurement ismade; determining any deviation between said values; and calibrating thesensor to account for any such deviation.
 10. A method as claimed inclaim 9, in which the step of calibrating the sensor comprises:progressively varying the proportion of the calibrating gas in thecalibrating gas mixture directed to the sensor between the firstproportion and the second proportion; monitoring the sensor outputduring the period that the proportion of said calibrating gas is varied;comparing the partial pressure values of the calibration gas outputtedby the sensor to the actual partial pressure values which the sensorshould output for said proportion of calibration gas at the pressure inthe pressure vessel at which the measurement is made; determining anydeviation between said values; and calibrating the sensor to account forany such deviation.
 11. A method as claimed in claim 9, in which twoseparate sources of pre-prepared mixtures of low point and high pointcalibrating gas mixtures are directed to the sensor.
 12. A method asclaimed in claim 11, in which the proportion of calibrating gas in thecalibrating gas mixture is varied by controlling the flow of low andhigh point calibrating gas mixtures to the sensor.
 13. A method asclaimed in claim 8, in which the calibration gas is O₂, and in which thefirst proportion of O₂ in the low point calibration gas mixture isselected so that the partial pressure of the gas in the calibration gasmixture, at the pressure in the vessel, is no less than around 0.15 bar.14. A method as claimed in claim 13, in which the second proportion ofO₂ in the high point calibration gas mixture is selected so that thepartial pressure of the gas in the calibration gas mixture, at thepressure in the vessel, is no more than around 0.60 bar.
 15. A methodpositioning a gas analysis sensor within the pressure vessel; exposingthe sensor to the mixture of gasses at the pressure level found in thepressure vessel; operating the sensor to measure the actual partialpressure of the at least one gas in the mixture contained in the vessel;and periodically calibrating the sensor by directing a calibrating gasmixture to the sensor in the vessel, the calibrating gas mixture beingbreathable by a human being, in which the step of calibrating the sensorcomprises directing a pre-prepared calibration gas mixture to thesensor, said pre-prepared mixture comprising a determined proportion ofa calibrating gas.
 16. A method of determining the partial pressure ofat least one gas in a mixture of gasses contained in a pressure vessel,the mixture being pressurised to a level which is above localatmospheric pressure, in which the method comprises the steps of:positioning a gas analysis sensor within the pressure vessel; exposingthe sensor to the mixture of gasses at the pressure level found in thepressure vessel; operating the sensor to measure the actual partialpressure of the at least one gas in the mixture contained in the vessel;periodically calibrating the sensor by directing a calibrating gasmixture to the sensor in the vessel, the calibrating gas mixture beingbreathable by a human being; wherein the step of positioning the sensorwithin the vessel comprises locating the sensor within a containerdefining a sensor chamber, and arranging the sensor chamber so that itcommunicates with the mixture of gasses in the pressure vessel; andflooding the sensor chamber with the calibration gas mixture to expelvessel mixture from the sensor chamber.
 17. A method as claimed in claim16, comprising venting the calibrating gas mixture into the pressurevessel.
 18. A method as claimed in claim 16, comprising directing thecalibrating gas mixture into the vessel along a conduit having an outletwhich communicates with the sensor chamber, and arranging the outlet sothat it directs the mixture towards an inlet of the sensor.
 19. A methodas claimed in claim 18, comprising arranging the outlet so that itdirects a jet of calibrating gas mixture towards the sensor inlet.
 20. Amethod as claimed in claim 16, wherein the step of calibrating thesensor comprises directing a pre-prepared calibration gas mixture to thesensor, said pre-prepared mixture comprising a determined proportion ofa calibrating gas.